diff --git a/.gitignore b/.gitignore
index 58f394d..6987955 100644
--- a/.gitignore
+++ b/.gitignore
@@ -1,2 +1,4 @@
 /vendor/
 .DS_Store
+.phpunit.result.cache
+
diff --git a/src/IndieAuth/Client.php b/src/IndieAuth/Client.php
index 5945faf..bb783f3 100644
--- a/src/IndieAuth/Client.php
+++ b/src/IndieAuth/Client.php
@@ -236,11 +236,37 @@ private static function _isIssuerValid($issuer, $metadata_endpoint) {
 
     $metadata_endpoint = self::normalizeMeURL($metadata_endpoint);
 
-    if (strpos($metadata_endpoint, $issuer) !== 0) {
+    // The issuer is a prefix of the metadata URL (IndieAuth), or the metadata
+    // URL is the RFC 8414 well-known location for the issuer, which inserts
+    // /.well-known/oauth-authorization-server between the host and the
+    // issuer's path. The next IndieAuth draft uses the RFC 8414 form; servers
+    // such as IndieKey.id already do.
+    if (strpos($metadata_endpoint, $issuer) === 0) {
+      return true;
+    }
+
+    return rtrim($metadata_endpoint, '/') === self::wellKnownMetadataURL($issuer);
+  }
+
+  /**
+   * The RFC 8414 (section 3) metadata location for an issuer identifier:
+   * https://host/.well-known/oauth-authorization-server followed by the
+   * issuer's path. Without a trailing slash, for comparison. False when the
+   * issuer is not a URL.
+   *
+   * @param string $issuer a normalized issuer URL
+   * @return string|false
+   */
+  public static function wellKnownMetadataURL($issuer) {
+    $parts = parse_url($issuer);
+    if(!is_array($parts) || !isset($parts['scheme'], $parts['host'])) {
       return false;
     }
 
-    return true;
+    $base = strtolower($parts['scheme']) . '://' . strtolower($parts['host']) . (isset($parts['port']) ? ':' . $parts['port'] : '');
+    $path = rtrim($parts['path'] ?? '', '/');
+
+    return $base . '/.well-known/oauth-authorization-server' . $path;
   }
 
   private static function _fetchHead($url) {
diff --git a/tests/MetadataTest.php b/tests/MetadataTest.php
index 10182fb..16bf06f 100644
--- a/tests/MetadataTest.php
+++ b/tests/MetadataTest.php
@@ -128,6 +128,66 @@ public function testIsIssuerValid() {
     );
 
     $this->assertTrue($result);
+
+    # RFC 8414: the well-known path is inserted between the host and the issuer's path
+    $result = $this->_invokeStaticMethod(
+      Client::class,
+      '_isIssuerValid',
+      ['https://example.com/s/abc/', 'https://example.com/.well-known/oauth-authorization-server/s/abc']
+    );
+
+    $this->assertTrue($result);
+
+    # OpenID Connect discovery has its own rules and is not IndieAuth metadata
+    $result = $this->_invokeStaticMethod(
+      Client::class,
+      '_isIssuerValid',
+      ['https://example.com/s/abc/', 'https://example.com/.well-known/openid-configuration/s/abc']
+    );
+
+    $this->assertFalse($result);
+
+    # ... and for an issuer without a path (also a prefix match)
+    $result = $this->_invokeStaticMethod(
+      Client::class,
+      '_isIssuerValid',
+      ['https://example.com/', 'https://example.com/.well-known/oauth-authorization-server']
+    );
+
+    $this->assertTrue($result);
+
+    # the well-known location for a different path is not this issuer's
+    $result = $this->_invokeStaticMethod(
+      Client::class,
+      '_isIssuerValid',
+      ['https://example.com/s/abc/', 'https://example.com/.well-known/oauth-authorization-server/s/xyz']
+    );
+
+    $this->assertFalse($result);
+
+    # nor is the well-known location on another host
+    $result = $this->_invokeStaticMethod(
+      Client::class,
+      '_isIssuerValid',
+      ['https://example.com/s/abc/', 'https://example.org/.well-known/oauth-authorization-server/s/abc']
+    );
+
+    $this->assertFalse($result);
+
+    # http is still refused, well-known or not
+    $result = $this->_invokeStaticMethod(
+      Client::class,
+      '_isIssuerValid',
+      ['http://example.com/s/abc/', 'http://example.com/.well-known/oauth-authorization-server/s/abc']
+    );
+
+    $this->assertFalse($result);
+  }
+
+  public function testWellKnownMetadataURL() {
+    $this->assertEquals('https://example.com/.well-known/oauth-authorization-server/s/abc', Client::wellKnownMetadataURL('https://example.com/s/abc/'));
+    $this->assertEquals('https://example.com:8443/.well-known/oauth-authorization-server', Client::wellKnownMetadataURL('https://example.com:8443/'));
+    $this->assertFalse(Client::wellKnownMetadataURL('not a url'));
   }
 
   public function testDiscoverIssuer()
@@ -175,6 +235,22 @@ public function testDiscoverIssuerNotAPrefix()
     $this->assertInstanceOf(ErrorResponse::class, $result);
   }
 
+  /**
+   * `issuer` may also name the RFC 8414 well-known location the metadata was fetched from
+   */
+  public function testDiscoverIssuerAtWellKnownLocation()
+  {
+    $metadata_endpoint = 'https://example.com/.well-known/oauth-authorization-server/s/abc';
+    Client::setMetadata($metadata_endpoint, '{"issuer":"https://example.com/s/abc/","authorization_endpoint":"https://example.com/s/abc/auth","token_endpoint":"https://example.com/token"}');
+
+    $result = $this->_invokeStaticMethod(
+      Client::class,
+      'discoverIssuer',
+      [$metadata_endpoint]
+    );
+    $this->assertEquals('https://example.com/s/abc/', $result);
+  }
+
   public function testDiscoverRevocationEndpoint() {
     Client::setMetadata('https://example.com/', $this->default_metadata);